Hokuriku Shinkansen Tsuruga Extension Project Vol.3 ~ Kakehashigawa Bridge ~

INDEX

• A new challenge for river bridge
• In-depth analysis is needed to meet the sense of responsibility of river manager
• A reliable response: "Leave it to me"
• The thrill of leaving your mark on a major national project
• Project Overview

A new challenge for river bridge

Koji SHIBATA in Underground Structure I Sec. of Railway Dept. (then changed to Complex Project Sec.), who was in charge of designing the Kakehashi River Bridge in the fall of 2014, had been involved in the design of many viaducts and bridges for conventional railway lines, but this was his first experience designing a bridge for a proposed Shinkansen line. Moreover, this was his first detailed design of a river bridge with piers built over the riverbed of a first-class river, so this was a first challenge in two senses. SHIBATA was nervous but also excited about what he would learn.

However, his elation vanished when he first met with the river manager. SHIBATA realized that building sturdy bridge piers for the Shinkansen on the course of a Class A river was an entirely different endeavor from designing regular bridge piers built on the existing ground. The inclusion of the piers would change the flow of the water. What kind of change would that be? Would it increase the risk of flooding? The interest of the river manager, who was responsible for the lives of the residents in the river basin, was far greater than SHIBATA had imagined.

In-depth analysis is needed to meet the sense of responsibility of river manager

If the extension of the Shinkansen is a national project, the management of a first-class river is also an important task for the country. The Kakehashi River is originally characterized by its narrow width and rapid rise in water level when it rains. The densely populated urban area of Komatsu is located within the flood-prone area, and in fact, floods occurred in 2004, 2006, and 2013 even in the 2000s. Although river improvement work, including river channel excavation, is underway, the risk of flooding is still high, and it is a river that river manager cannot let their guard down over. Moreover, the proposed bridge construction site is in an area where the river meanders greatly, with waterways merging from both sides. In addition, a local high school has set up a boathouse and made part of the riverbank into a stepped structure so that boats can be launched. There are many elements that complicate the water flow, and the riverbank is widely used by citizens.

"It is natural that river manager would be concerned that the construction of new piers will impede the flow of water and increase the possibility of flooding. It would be best to not build any piers in the riverbed at all, but the planned crossing point is about 130 meters wide, and crossing this without any piers in the river would require a long structure of an unprecedented scale for the Shinkansen, and is not realistic. I thought that if we changed the plan from the standard three-span plan considered in the outline design to two spans, and reduced the number of piers entering the riverbed from two to one, and if we could present a detailed analysis of the hydraulic phenomena that would occur when this was done, the river manager would be more likely to agree. Local people have high hopes for the positive impact on tourism and revitalization of the region that would result from the extension of the Shinkansen and having it stop at Komatsu Station. I was sure we would be able to gain their understanding," said SHIBATA.

I immediately went to my company's Water Course Engineering & Water Resources Dept. to ask for cooperation.

A reliable response: "Leave it to me"

PACIFIC CONSULTANTS' Water Course Engineering & Water Resources Dept. formulates basic river improvement policies and plans for many first- and second-class rivers throughout the country, and is involved in various surveys and investigations related to flood control, water utilization, and the environment. They have a strong track record in hydraulic analysis, flood simulations, and river monitoring system construction, and are staffed with highly specialized engineers. While designing a two-span bridge instead of a three-span one, SHIBATA also requested the Water Course Engineering & Water Resources Dept. to carry a detailed hydraulic analysis, indicating the expected size and shape of the bridge pier for two cases, one with two piers and one with one. If a bridge pier is installed, it is natural that the water level will rise to a certain extent, the flow rate will change and become turbulent, and the soil on the riverbank and riverbed will be eroded, which is known as "scouring." Detailed analysis data on these cases was needed.

When SHIBATA explained the situation and his request to a veteran engineer in Water Course Engineering & Water Resources Dept., he received a reassuring response: "That's fine, leave it to me." Water Course Engineering & Water Resources Dept. will provide support. "It's really reassuring and appreciated to have a specialist engineer in our company. We had him attend meetings with the river manager, and he spoke from his perspective as a specialist engineer when explaining the analysis data we obtained and during question and answer sessions with the other party," said SHIBATA.

In fact, after discussing the project with the client, the Japan Railway Construction, Transport and Technology Agency (JRTT), and presenting detailed analysis data, they were able to gain the understanding of the river manager. What was required was a thorough explanation and procedure that was based on the perspective of the river manager, who bears the heavy responsibility of protecting the lives of residents. Additional, more advanced analysis was also required, and they would take that back, conduct further analysis and resubmit it over and over again, and after several meetings they were able to agree to a two-span bridge with an additional pier. "It was a relief," says SHIBATA.

"I think the analysis and simulations of Water Course Engineering & Water Resources Dept. were convincing. We proceeded with a full-scale detailed design based on the premise of a two-span structure."

Naturally, if the bridge had two spans, the distance between the piers would be longer and the girder height would be higher than in the original plan. In addition, the Kakehashigawa Bridge crosses the JR Hokuriku Main Line (now the IR Ishikawa Railway Line) about 200m after crossing the river in the direction of Komatsu Station, but since the JR Hokuriku Main Line is already elevated, it was necessary to run the bridge above it. We also proposed to the JRTT that the structural plan be revised to take into account the increased girder height at the crossing and the crossing over of the JR Hokuriku Main Line.

The impact on the river during the construction period also had to be carefully assessed. The impact on the river would not only occur after the bridge piers were completed. Temporary piers and scaffolding would be erected for a long period during the construction period. The impact on hydraulics from this also needed to be verified. This was also a concern for the river manager.

The bridge piers required were quite large, and the supporting layer at the site was 50m underground, so the foundation work was expected to be extensive. After considering several trial designs and construction plans for the foundation structure to be installed on the river channel, SHIBATA selected a construction method called steel pipe sheet pile well foundation, and held multiple meetings with Water Course Engineering & Water Resources Dept. based on this premise. He had Water Course Engineering & Water Resources Dept. carry out hydraulic analysis during construction, and after deciding on a policy with the JRTT using that data, he showed the river manager that safety could be ensured during the construction period. The foundation work was scheduled to be completed from October, when there is little water, known as the non-flood season, to May of the following year. Once the foundation work, including the pile work, was completed, the impact on the river during construction would be reduced.

The thrill of leaving your mark on a major national project

In total, there were around 10 discussions with the river manager regarding the bridge, and SHIBATA was responsible for the detailed design, which took about two years to complete, finishing in the fall of 2016.

"Even after completing the design, I visited the site many times to deal with changes in construction conditions and various issues that arose during construction. Each time, I had repeated discussions with the JRTT and the construction company, and overcame the issues by conducting additional studies, and these experiences have become a great asset to me. It is a great joy for me as an engineer to see my design through to completion, and to have been able to play a part in a major national project like the Hokuriku Shinkansen Extension Project. I learned a lot from this job, such as the strong desire and sense of responsibility of the river manager to protect the residents in the river basin, and the dependable support of Water Course Engineering & Water Resources Dept."

No two bridge designs are the same - SHIBATA's belief was strengthened after the project was completed. Everything is made to order, and the best design must be created for the location and environment, with the agreement of all parties involved. The process can be difficult at times, but the project cannot be successful without this effort.